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From: Loic Poulain <loic.poulain@linaro.org>
To: Stephan Gerhold <stephan@gerhold.net>
Cc: Bjorn Andersson <bjorn.andersson@linaro.org>,
	Aleksander Morgado <aleksander@aleksander.es>,
	Network Development <netdev@vger.kernel.org>,
	linux-remoteproc@vger.kernel.org,
	linux-arm-msm <linux-arm-msm@vger.kernel.org>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Ohad Ben-Cohen <ohad@wizery.com>,
	Mathieu Poirier <mathieu.poirier@linaro.org>
Subject: Re: [RFC] Integrate RPMSG/SMD into WWAN subsystem
Date: Mon, 7 Jun 2021 11:27:07 +0200	[thread overview]
Message-ID: <CAMZdPi_-Qa=JnThHs_h-144dAfSAjF5s+QdBawdXZ3kk8Mx8ng@mail.gmail.com> (raw)
In-Reply-To: <YLtDB2Cz5ttewsFu@gerhold.net>

Hi Stephan,

On Sat, 5 Jun 2021 at 11:25, Stephan Gerhold <stephan@gerhold.net> wrote:
>
> Hi Loic,
>
> On Fri, Jun 04, 2021 at 11:11:45PM +0200, Loic Poulain wrote:
> > On Wed, 2 Jun 2021 at 20:20, Stephan Gerhold <stephan@gerhold.net> wrote:
> > > I've been thinking about creating some sort of "RPMSG" driver for the
> > > new WWAN subsystem; this would be used as a QMI/AT channel to the
> > > integrated modem on some older Qualcomm SoCs such as MSM8916 and MSM8974.
> > >
> > > It's easy to confuse all the different approaches that Qualcomm has to
> > > talk to their modems, so I will first try to briefly give an overview
> > > about those that I'm familiar with:
> > >
> > > ---
> > > There is USB and MHI that are mainly used to talk to "external" modems.
> > >
> > > For the integrated modems in many Qualcomm SoCs there is typically
> > > a separate control and data path. They are not really related to each
> > > other (e.g. currently no common parent device in sysfs).
> > >
> > > For the data path (network interface) there is "IPA" (drivers/net/ipa)
> > > on newer SoCs or "BAM-DMUX" on some older SoCs (e.g. MSM8916/MSM8974).
> > > I have a driver for BAM-DMUX that I hope to finish up and submit soon.
> > >
> > > The connection is set up via QMI. The messages are either sent via
> > > a shared RPMSG channel (net/qrtr sockets in Linux) or via standalone
> > > SMD/RPMSG channels (e.g. "DATA5_CNTL" for QMI and "DATA1" for AT).
> > >
> > > This gives a lot of possible combinations like BAM-DMUX+RPMSG
> > > (MSM8916, MSM8974), or IPA+QRTR (SDM845) but also other funny
> > > combinations like IPA+RPMSG (MSM8994) or BAM-DMUX+QRTR (MSM8937).
> > >
> > > Simply put, supporting all these in userspace like ModemManager
> > > is a mess (Aleksander can probably confirm).
> > > It would be nice if this could be simplified through the WWAN subsystem.
> > >
> > > It's not clear to me if or how well QRTR sockets can be mapped to a char
> > > device for the WWAN subsystem, so for now I'm trying to focus on the
> > > standalone RPMSG approach (for MSM8916, MSM8974, ...).
> > > ---
> > >
> > > Currently ModemManager uses the RPMSG channels via the rpmsg-chardev
> > > (drivers/rpmsg/rpmsg_char.c). It wasn't my idea to use it like this,
> > > I just took that over from someone else. Realistically speaking, the
> > > current approach isn't too different from the UCI "backdoor interface"
> > > approach that was rejected for MHI...
> > >
> > > I kind of expected that I can just trivially copy some code from
> > > rpmsg_char.c into a WWAN driver since they both end up as a simple char
> > > device. But it looks like the abstractions in wwan_core are kind of
> > > getting in the way here... As far as I can tell, they don't really fit
> > > together with the RPMSG interface.
> > >
> > > For example there is rpmsg_send(...) (blocking) and rpmsg_trysend(...)
> > > (non-blocking) and even a rpmsg_poll(...) [1] but I don't see a way to
> > > get notified when the TX queue is full or no longer full so I can call
> > > wwan_port_txon/off().
> > >
> > > Any suggestions or other thoughts?
> >
> > It would be indeed nice to get this in the WWAN framework.
> > I don't know much about rpmsg but I think it is straightforward for
> > the RX path, the ept_cb can simply forward the buffers to
> > wwan_port_rx.
>
> Right, that part should be straightforward.
>
> > For tx, simply call rpmsg_trysend() in the wwan tx
> > callback and don't use the txon/off helpers. In short, keep it simple
> > and check if you observe any issues.
> >
>
> I'm not sure that's a good idea. This sounds like exactly the kind of
> thing that might explode later just because I don't manage to get the
> TX queue full in my tests. In that case, writing to the WWAN char dev
> would not block, even if O_NONBLOCK is not set.

Right, if you think it could be a problem, you can always implement a
more complex solution like calling rpmsg_send from a
workqueue/kthread, and only re-enable tx once rpmsg_send returns.

>
> But I think you're right that it's probably easiest if I start with
> that, see if I can get anything working at all ...
>
> > And for sure you can propose changes in the WWAN framework if you
> > think something is missing to support your specific case.
> >
>
> ... and then we can discuss that further on a RFC PATCH or something
> like that. Does that sound good to you?

Yes, you can submit the series, no need to be RFC IMHO, this thread is
already your RFC.

Regards,
Loic

  reply	other threads:[~2021-06-07  9:18 UTC|newest]

Thread overview: 11+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-06-02 18:20 Stephan Gerhold
2021-06-04 21:11 ` Loic Poulain
2021-06-05  9:25   ` Stephan Gerhold
2021-06-07  9:27     ` Loic Poulain [this message]
2021-06-07 10:54       ` Stephan Gerhold
2021-06-07 11:23         ` Toke Høiland-Jørgensen
2021-06-07 11:44           ` Stephan Gerhold
2021-06-07 12:16             ` Loic Poulain
2021-06-07 12:48               ` Stephan Gerhold
2021-06-15  9:03                 ` Loic Poulain
2021-06-08 11:30             ` Toke Høiland-Jørgensen

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